This invention belongs to a technical field of interleave recording where a multi-channel recording head is used to perform image recording in a higher resolution than the physical resolution of a recording head, and in particular relates to a multi-channel recording head which is capable of performing image recording as usual without causing a drop in the productivity even in case the recording head includes a faulty channel, an recording method using this recording head and an image recording apparatus which executes this image recording method.
A multi-channel recording head having a plurality of recording channels (recording elements) has been used in a variety of printers, such as a multi-channel exposure head which uses a plurality of light beams to expose a photosensitive material to light and an ink-jet recording head where a plurality of nozzles for discharging ink droplets are arranged.
As one of the image recording methods using such a multi-channel recording head (hereinafter simply referred to as a recording head), a so-called image recording method by way of a drum scanner is known where a recording medium such as a photosensitive material or image receiving paper is wrapped around the periphery of a drum to fix itself onto the drum, and by moving a recording head in the direction of the axis of a drum (sub-scanning) while rotating the drum (main scanning) and having the direction of the recording channel array on the recording head aligned with the direction of the axis of the drum, the recording material is scanned two-dimensionally.
In image recording by way of a drum scanner using such a recording head, the interleave (image) recording is known where images are recorded by driving each recording channel so as to bridge the gaps between adjacent recording channels in accordance with the travel amount of the recording head in the direction of the axis of the drum thereby performing image recording at a higher resolution than that of the recording head (physical resolution of the recording head).
As an example of interleave recording by way of a multi-channel recording head, a recording method shown in FIG. 6 is known.
The example shown performs image recording at 600 dpi by using a recording head having four channels (circles with solid lines in the figure) whose resolution is 150 epi (ejection per inch). In this image recording method, four-fold image recording is made with interleave recording. For each rotation of the drum, the recording head moves, by one quarter of its channel pitch, in the direction of the axis (array of recording channels) shown by an arrow to record an image.
With four rotations of the drum in the image recording, that is, four times of image recording, the respective gaps between recording channels are filled with three recording pixels. This provides image recording at 600 dpi.
In this recording method, in order to perform recording in the downstream of the pixels already recorded, that is, in the travel direction of the recording head, the recording head is moved before recording with the fifth rotation in one stroke until the recording channel extremely upstream is 600 dpi away from the recording position of the recording channel extremely downstream at the end of the fourth rotation, as shown by an arrow m. Next, recording for four rotations of the drum is performed similarly. Then, the recording head is moved in one stroke to perform the next recording. This procedure is repeated to record an image at 600 dpi on the entire surface of a recording medium by using a four-channel recording head whose resolution is 150 epi.
However, such a recording method (step-scan) has a problem that a recording head must be moved by a substantial amount per predetermined number of drum rotations to correspond to an improved resolution by interleave recording and this tends to result in uneven image recording.
Interleave recording to solve this problem is also known which employs a so-called helical scanning while rotating a drum and continuously moving a recording head at a constant speed to record an image on the entire surface of a recording medium. In this method, image recording is performed where the ratio of travel amount of the recording head per rotation of the drum to the channel pitch (1 pitch) of the recording channel is assumed as p, the ratio of the recording resolution to the physical resolution of the recording head as N, the number of recording channels as M, and an arbitrary integer as X, N and M being integers and the following expressions (1) and (2) are satisfied (detailed later):
p*N=Mxe2x80x83xe2x80x83Expression (1)
p=X+1/Nxe2x80x83xe2x80x83Expression (2)
FIG. 7 shows an example of the recording method.
The example shown in FIG. 7 performs image recording at 600 dpi by using a recording head having five recording channels whose resolution is 150 epi each. In this example, the recording head is continuously moved at a ratio of 1.25 to the pitch of the recording channel per rotation of the drum to perform image recording with each recording channel.
In this way, as shown in FIG. 7, on and after the fourth rotation (from the midst of the third rotation), each gap between recording channels is filled with three recording pixels. This provides image recording at 600 dpi by using a recording head having a resolution of 150 epi.
In the interleave recording using this recording head, when a channel goes faulty (defective), a line-shaped whiteness clarity or streaked unevenness appears on the recorded image, an image of a proper quality cannot be recorded.
In order to prevent such an inconvenience, various methods have been proposed.
One of the methods is to group odd-numbered channels of a recording head and even-numbered channels of the recording head, and in case a faulty channel is detected, the recording channels of a group which does not contain the faulty channel are used to record an image.
In the method, however, the resolution of the resulting recorded image after a channel has gone faulty is reduced to half the normal resolution. To record an image in the same resolution as that before a channel has gone faulty, the productivity is reduced by half.
Another method is known where the array of recording channels is split into two arrays at the faulty channel. The array which has the larger number of channels is used to perform image recording.
In this method, the number of channels of the recording head differs in accordance with the location of the faulty channel, so that the productivity differs case by case. Moreover, image processing such as assignment of image data is complicated thus resulting in higher system costs.
A method is also known where two recording heads are used or a spare channel same as a recording channel is used. In this method, the cost of the recording head is nearly doubled. A room where the spare head is to be arranged is required. Further, the size of the recording head is doubled. These disadvantages limit the freedom of apparatus design and prevents downsizing of the apparatus.
The object of the invention is to solve the problems of the related art and to provide a multi-channel recording head which is capable of performing image recording without lowering the productivity and resolution in a short halt period and low cost even in the presence of a faulty channel in the interleave recording to provide image recording at a higher resolution than the physical resolution of a recording head, by employing a drum scanner using a multi-channel recording head thus filling the gaps between recording channels with pixels in image recording, and which is capable of preventing reduction of an yield due to the faulty channel, an image recording method using the recording head, and an image recording apparatus which executes this image recording method.
In order to attain the object described above, the present invention provides a multi-channel recording head the physical resolution of which is lower than the resolution of an image to be recorded, comprising: a plurality of recording channels arranged in one direction; and spare channels arranged away from said recording channels by an integral multiple of the channel pitch of said recording channels on the extension of an array of said recording channels, said spare channels being arranged as least as many as the number of said recording channels, and each spare channel having the same physical resolution as the resolution of said image to be recorded.
Preferably, said resolution of said image to be recorded is the dot resolution of said image to be recorded.
Preferably, said spare channels are arranged on both ends of the array of said recording channels.
In order to attain the object described above, the present invention provides an image recording method of recording an image by using the recording head described above, said image recording method comprising: rotating a drum with a recording medium wrapped around the periphery of the drum; moving said recording head in a direction of an axis of the drum while aligning a direction of the array of said recording channels of said recording head with the direction of the axis of the drum; and modulating each recording channel of said recording head in accordance with said image to be recorded, thereby performing image recording at a higher resolution than said physical resolution of said recording head, when a faulty channel exists among said recording channels, said image recording method further comprising: previously determining a spare channel corresponding to the faulty channel among said recording channels; assigning recording data on said fault channel to the corresponding spare channel in the image recording; and modulating said corresponding spare channel with said assigned recording data in accordance with the number of rotations of said corresponding spare channel behind said faulty channel in the image recording.
In order to attain the object described above, the present invention provides an image recording apparatus, comprising: the recording head described above; a drum rotating with a recording medium wrapped around the periphery of the drum; moving means for moving said recording head in the direction of the axis while aligning the direction of the array of the recording channels of said recording head with the direction of the axis of the drum; determining means for acquiring the information on a faulty channel of said recording head and determining a spare channel corresponding to the faulty channel; and modulating means for modulating each recording channel of said recording head in accordance with the rotation of said drum and travel of said recording head, and when the faulty channel exists among said recording channels, assigning the recording data on said fault channel to the corresponding spare channel, and modulating said spare channel with said assigned recording data in accordance with the number of rotations of the spare channel behind the faulty channel, thereby performing image recording at a higher resolution than the physical resolution of said recording channel.
In the image recording method and the image recording apparatus of the present invention, the ratio of travel amount of the recording head per rotation of said drum to the channel pitch of said recording channel is assumed as p, the ratio of the recording resolution to the physical resolution of said recording head as N, the number of recording channels of said recording head as M, and an arbitrary integer as X, N and M being integers and the following expressions (1) and (2) are satisfied:
p*N=Mxe2x80x83xe2x80x83Expression (1)
p=X+1/Nxe2x80x83xe2x80x83Expression (2)